Effects of a storm event on the structure of the pelagic food web with special emphasis on planktonic ciliates

Storm-induced changes in the water column structure and thepelagic food web were investigated by daily sampling duringa 3 week period (October 24 to November 10, 1988) at a permanentstation in the southern Kattegat (Denmark) Subsequent to a stormthe primary production increased and the size-distribution ofthe phytoplankton changed towards larger cells. Some componentsof the zooplankton community responded to the increase in potentialfood, either in terms of a functional response (copepods) orin terms of a numerical response (cladocera), whereas no changesin the populations of mixo- and heterotrophic aliates were evident.The dilates were not food limited since growth rates determinedin situ were comparable to maximum growth rates measured inthe laboratory. Calculations suggest that the total clearancecapacity of planktonic copepods (as fraction of water columncleared per unit time) was of the same magnitude as the measuredinstantaneous growth rates of the ciliates (0.55–0 85day^–1). Ciliate populations were therefore probably limitedby copepod predation. Ciliates, however, contributed only insignificantlyto the diets of copepods. Although the total zooplankton productionincreased by at least 30% subsequent to the storm (and thatof the copepods by 50%), the zooplankton community was unableto assimilate the entire increase in primary production. Therelative zooplankton (copepods, cladoceran and ciliates) grazingrate on the primary production decreased 30%, and the flow ofcarbon to biomass accumulation, other grazers and in particularsedimentation is predicted to have increased by 100% followingthe storm.